<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    // if  ... else  if...
    var age = parseInt(prompt('请输入您的年龄'))

    if(age >= 18) {
      alert('可以取洗脚')
    } else {
      alert('可以去网吧')
    }
  </script>
</body>
</html>
